1. Field of the Invention
In a particular application of the method of the present invention, seismic data is processed to remove or suppress data produced by multiple reflections from the same subsurface reflector. The method may be employed to enhance data of any type which contains a prime event and a multiple of the prime where the multiple exhibits a discernable periodicity.
2. Brief Description of the Prior Art
In seismic data and in other types of data there are multiple events produced by seismic waves which have traveled two or more times through the shallow formations and arrived at later times on the records. Under certain conditions which are found in many areas, the amplitudes of the multiple events are large enough to interfere with the prime events and, in some cases may even become larger in amplitude than the prime. Under any of these conditions, the prime events are obscured by the interference of the multiple events and the prime events may be completely masked by the multiple events when the amplitude of the multiples is large.
One major problem in dealing with multiple reflections stems from the fact that the period of the multiples may vary along the trace. As a result of this variance, multiple elimination processes which employ the average of the multiple period will have differing effectiveness depending upon the range of departure of the periods from the average.
The causes for the changing periods of the multiple events are many. A major cause is the difference in the length of the travel path for the seismic wave during the prime or initial reflection and subsequent reflections. For example as in marine seismic recording, if the source and detector are below the earth's surface, the prime reflection path length is directly from the source to the reflector and back up to the detector. The returning wave, however, travels upwardly beyond the detector and is reflected back down from the interface at the surface. The subsequent upward reflection of the same wave from the same subsurface reflector produces a multiple when it is picked up by the detector. The travel time of the multiple is different from that of the prime by the amount of time required to travel from the source to the surface and back down to the detector. Subsequent reflections of the same wave may also travel different path lengths so that the travel time of the multiples may be different from each other as well as from the prime.
Other situations, including a dipping reflector, may produce deviations from the exact integral multiples of the travel times of successive reflections of the same wave. The net result is that the times of appearance of multiples on the seismic records often deviate from exact, predictable times derived from the measured travel times of prime events through the layers producing multiples.
Several techniques including visual inspection have been employed to remove and suppress multiples in the record. One technique employs velocity or moveout filters which discriminate between signal and noise on the basis of a difference in the moveout or apparent velocity of coherent events across an array of seismic traces. In another prior art technique, synthetic multiple events are produced by a correlation technique and the synthetic multiples are then visually compared with the actual trace to assist in locating prime events. It has also been suggested to suppress the multiples by converting the traces into their real and imaginary parts by Fourier transformation. The two parts of the trace are then employed to form operators which may be used to remove distortion components in the trace.
Yet another technique for multiple elimination employs "inverse convolution filtering" or deconvolution filtering in which a linear prediction error filter having an impulse time response approximately the inverse of the multiple is applied to the seismic trace. Various other techniques have also been proposed and employed with differing degrees of success.